1. Field of the Invention
The present invention relates to a method of examining an internal structure, such as a welding state and an internal defect, of a magnetic body like a steel plate in a non-destructive manner, as well as to an apparatus for the same.
2. Description of the Related Art
Spot welding is generally used in the automobile industry and for assembly of various metal thin plate products. The typical procedure of spot welding pinches layers of a metal base material between the appropriately formed ends of electrodes. The process then concentrates the electric current and the pressing force on a relatively narrow portion and thereby locally heat the relatively narrow portion, while applying the pressure to the relatively narrow portion with the electrodes. This is a sort of resistance welding.
The spot welded part generally has a sectional structure shown in FIG. 19. The surface of the welded part is concaved (indentation) by the pressure to be lower than the surrounding non-welded part. The dimension of the indentation is referred to as the indentation diameter. The welded part includes a nugget (deposit), which is the center of the welded part, and a circumferential contact-bonded portion. The nugget is formed by solidification of the fused metal. The contact-bonded portion is formed by bonding of the surfaces of the metal layers which are in closed contact with each other. The dimension of the nugget is referred to as the nugget diameter. The dimension of the sum of the nugget and the contact-bonded portion (that is, the actually joint part) is referred to as the joint diameter. Since the spot welding welds the layers of the metal base material at one point, it is often followed by an examination to check whether the welding strength is sufficient or not.
One effective method of measuring the welding strength in a non-destructive manner measures the nugget (deposit) diameter of the welded part to determine the welded strength. A known method of measuring the nugget diameter applies an ac magnetic field, which is produced by a coil through which a high-frequency electric current flows, to the spot welded part and measures a resulting variation in inductance of the coil to determine the nugget diameter. The conventional method takes advantage of the characteristic that the permeability is different between the nugget and the non-nugget portion, and observes a variation in permeability as a variation in inductance to determine the nugget diameter.
The indentation has a less thickness, and the indentation diameter thereby affects the structural strength. The nugget diameter and the joint diameter, on the other hand, affect the joint strength. In order to determine the welding strength of the spot welded part with high accuracy, it is desirable to determine all the indentation diameter, the nugget diameter, and the joint diameter.
The conventional method, however, measures only the nugget diameter and does not determine the other structural characteristic values, such as the indentation diameter and the joint diameter. There has been a strong demand or the technique that measures a variety of internal structures of not only the spot welded part but a general magnetic body in a non-destructive manner.